Sex Ed
by AnimaAmore
Summary: Naruto's failing biology class, and with a big test on the horizon he begs Sakura to help him. When she agrees, it turns into quite an experience, as Sasuke can tell you. After all, it's not every day you see temperamental Sakura teaching the prankster knucklehead about sex ed. AU two-shot
1. Chapter 1 - Bio SOS

**Author:** AnimaAmore

 **Rating:** T

 **Genre(s):** General, comedy, friendship, informative

 **Fanfic type:** High school AU, two-shot

 **Pairing:** SasuSakuNaru, Team 7 friendship, slight implied NaruHina

 **Disclaimer:** Characters belong to Masashi Kishimoto. This story, however, is mine.

 **Author's comments:** this idea came to me when I imagined what it'd be like to try to explain to someone the finer points of what is known as human reproduction, particularly when it is expected that they already know. Since the explanation can drag on depending on how in-depth the explanation is desired to be, I'm settling for explaining just the menstrual cycle (aka, periods) since that is what seems more likely to come up in a conversation.

 **Summary:** Naruto doesn't particularly dislike school. He simply doesn't particularly like studying, especially things that he believes he won't have any use for in the future. But with a big biology test in the horizon he finally begs his friend Sakura to help him out. She agrees. And Sasuke? Well, he's just along for the ride. After all, it's not every day that the blonde knucklehead has Sex Ed. shoved down his throat by their temperamental friend.

* * *

"This sucks!" Naruto wailed as he dropped his head onto his desk with a resounding smack.

Sasuke, his best friend and long time rival, only gave him an annoyed side glance before meticulously tucking in the quizzes they'd just gotten back into his folder. He scoffed at the dramatic blonde. "If you'd studied then you wouldn't be in this position," he told him bluntly.

Naruto narrowed his azure-blue eyes at the raven-haired teen. "Yeah, well, if you're so smart then why didn't you get a hundred?"

"I did."

"Come again?"

Rolling his eyes, Sasuke stood up and swung his backup over one shoulder. "I did," he repeated. "Honestly, dobe, at least half of what was on that quiz was from previous years. How did you ever pass middle school?"

Scowling, Naruto grabbed his odious quiz and stuffed it in his bag before following Sasuke out into the hall. He didn't care if it was rumpled or ripped; in fact, he'd be happy to burn it. "Just because I'm not a straight-A student like Mr. High and Mighty here doesn't mean I'm incapable of learning."

Sasuke raised his eyebrows at him in mock surprise. "I'm astounded you know the meaning of 'incapable.'"

Flushing, partly from embarrassment an partly from anger, Naruto growled, "Why you –" as he reached for Sasuke's shirt, only to be intercepted by a feminine hand grabbing his and forcefully yanking it down to his side.

"Sakura-chan!" Naruto exclaimed, anger forgotten as he greeted the final piece of their tight trio of friendship, and the only girl, who easily gave Sasuke a run for his money in the brains department.

"Hey guys," she easily greeted the two boys as she released the blonde's hand and smoothly fell into step between them. "What are you two arguing about now?"

"The dobe messed up a quiz and he's mad about it," Sasuke explained as he guided their little group towards their lockers.

"No," Naruto countered, "I'm angry because the teme is acting like a bastard."

Long used to reading between the lines when it came to her two friends, Sakura sighed as she tucked a pink lock behind her ear and adjusted her hair bun. "So, Naruto got a bad mark and Sasuke is antagonizing him about it," she stated more than asked.

The two boys looked at each other for a moment and shrugged; Sasuke indifferently and Naruto moodily. Seeing as that was as much affirmation as she'd ever get given the less than cheerful attitude of either of them, Sakura settled for simply smiling to herself and opening her locker. Both she and Naruto had been ecstatic when they learned they could choose their own lockers that year and had managed to rope Sasuke into choosing their three lockers to be side by side.

"So what subject was the quiz on?" she asked conversationally as she dug out her agenda from her bag and skimmed over her what she needed from her locker to do her homework that day.

"Biology," Sasuke answered beside her as he exchanged a pair of textbooks between his bag and locker. Naruto scoffed, stuffing his lunch bag into his backpack from Sakura's other side.

"It's a waste of time," he grumbled. "I'm not interested in any of that. Sakura's the one who wants to be a doctor! Frankly I couldn't care less. I don't see when I'm ever going to need this in my life as a professional food taster."

"I'm still sure that that's not a real job," Sasuke quipped.

"Shut up."

"Why are you taking biology, again?" Sakura asked, closing her locker. Naruto and Sasuke did the same as they followed her towards the stairs to the first floor.

"Because my godfather, Jiraiya, insisted," Naruto huffed in annoyance. He made a face as if he'd just tasted something rancid. "Just because I don't get queasy at the mere thought of blood and guts, and because apparently knowing some biology is good for _everyone_."

"Considering how immensely complicated our bodies are, I think having a basic understanding is good for everyone," Sakura said. "It gives you a greater appreciation for them. Besides, if you recall from our history class, when people didn't know how bodies worked they assumed any injury or illness could be cured with pig poop, a random mix of herbs and chanting."

Naruto shivered to show his disgust. "Yeah, those were the good old days, alright."

"I'm sure you'd have survived just fine back then," Sasuke mused as they exited the school. He held the door open for a pair of girls who were walking right behind them, and promptly ignored their swooning and tittering giggling as he turned his back on them and kept walking. "You're too stupid to catch anything contagious, but at least you're fit enough to avoid any serious injuries."

Naruto, for his part, didn't know if to be deeply offended or somewhat pleased by the backhanded praise, since Sasuke rarely said anything remotely nice. Before he could decide whether to pick a fight with the brunette or just ignore him, Sakura once again came to the rescue by cutting in.

"Alright, that's enough of you," she said, giving Sasuke a pointed stare. He _humph'd_. Turning back to Naruto, she asked, "What topic are you guys studying?"

Yawning, Naruto made a half-hearted effort to cover his mouth as he answered. "Human reproduction."

Sakura pursed her lips. "Considering how many years of sex ed. we've had, it shouldn't be that hard."

"That's what the teme said earlier!"

"That just reinforces the truth of my words, not that they were wrong to begin with," Sasuke smirked. Sakura discretely elbowed him on the ribs, earning her an annoyed glare, which she ignored.

"How badly did you do on your quiz?" she asked.

"Got a 59," he quietly admitted, shame-faced. Beside him, Sasuke just shook his head and Sakura looked pensively at the blonde. Seeing their expressions, Naruto quickly defended himself. "But I already know all that I need to know, dattebayo! Some dude and some chick have unprotected sex and voila, they have a mini me in nine months! And if one of them has an STI, then the other person gets it too. It's not rocket science!"

"No, it's not," Sakura agreed soothingly. Her keen hearing caught Sasuke scoffing to the side, so instead of elbowing him again, she tripped him. As he'd turned his head, she got him that time, although she only managed to get him unbalanced for a moment before he caught himself and threw her a dirty look. She ignored it. As Naruto's friends they were supposed to be supportive, not condescending, damn it.

"But try to think of it as advancing our civilization," she continued, trying to get Naruto to appreciate at least an iota of what he was taught. "Not only were remedies extremely disgusting and inappropriate, not to mention downright toxic in some cases, but in the past since people didn't know what was happening in their own bodies they were embarrassed and did extreme things. After millennia of treating what is perfectly normal as taboo, many people refused to talk about it and got really sick or became outcasts when all they needed was a bit of understanding. I mean, can you imagine you guys getting accused of infidelity because you got boners at an inopportune moment? And let me remind you that in some cultures it wasn't just the women who got sentenced to death for that."

"S-Sakura-chan…" Naruto whined, his face slightly flushed. He loved Sakura dearly, truly he did, but the girl he saw practically as a sister really had a knack for talking about embarrassing things without batting an eye.

She pointed a finger at his red cheeks. "See, that right there is what I'm talking about. Why are you embarrassed? Because that's what we've been taught indirectly by society, which is only just learning to embrace science. Nowadays, with the education we have available, we know that guys get boners because of hormones which cause blood to accumulate in the penis – a natural reaction, especially during puberty, by the way – not because they are unfaithful, horny dogs or because they got cursed by some spirit or other."

"Sakura…" Sasuke tried to intervene, but he had no words to say. She was talking simply and straight to the point, and only speaking the truth. Besides, it wasn't like she was talking loudly and making a commotion. Still, although it wasn't as awkward and uncomfortable as he initially thought it'd be talking about that particular subject with a girl, it didn't mean he was perfectly at ease either.

Seemingly getting the hint, Sakura sighed and deflated a bit, resigned to containing herself. She had the bad habit of rambling on once she got started on a subject she was passionate about. "My bad. But you get the point. It's not always about proving that you memorized the textbook, but knowing enough to be _understanding_ of what is really going on under our skins. Getting embarrassed by, and even shaming others because our own physiology, is downright stupid; especially considering that it is what's responsible for keeping us alive both as individuals and as a species. The rift between the sexes due to ages of sexual segregation, especially, is problematic. Why do boys make fun of girls for things beyond their control and vice versa? And it happens within the sexes too. It creates nothing but bad blood and even worse memories in an idiotic and completely unnecessary cycle of shame and negativity."

"And so, guru Sakura strikes again," Sasuke said with a little smirk, slowly clapping his hands at her little speech. Embarrassed, she flushed and ducked her head. Seeing that Sasuke was, for once, not really being an ass and appeared to be actually praising her, if in that slightly backhanded way he used, she grinned shyly at him, her lips slightly twisting on one end sardonically.

Naruto, for his part, merely crossed his arms over his chest as he looked skyward. Although Sakura did rant a bit, she did have a point. Honestly, as much as he thought gladiators were awesome, it couldn't have been much fun having some obscene concoction made of who knows what foul thing shoved down his throat or, as Sakura had so helpfully reminded him, being banished or even executed just because nobody knew what was truly going on. In that sense, Naruto could see why having some basic understanding of biology could come in handy.

Especially when he thought about the needless cycle of shame passed down through generations all due to millennia-old misconceptions based on random beliefs.

Now _that_ was stupid.

 **~X~**

"Why meeee…?" Naruto bemoaned as he dropped face-down beside Sakura on the grass.

It was after school the following day, and his biology teacher, Mr. Ebizu, had dropped the big bomb on the class earlier: there would be a test next Monday worth a decent chunk of their final grade. Since it was halfway through the semester, Mr. Ebizu had decided that he would make a new unscheduled test that would be the same worth as the past three quizzes they'd had in class. If the students did well on the bigger test, he would discard the smaller the quizzes for their final grade.

Ebizu wasn't particularly famous for his generosity, so his students had jumped at the chance without hesitation, including Naruto. But of course, there had to be catch. It wasn't until the class had unanimously voted on taking the test that Ebizu had told them it would cover everything from the beginning of the term up until _next week's readings_. Considering Naruto hadn't even done the readings for the first week of class, he was certain he was royally screwed.

Sakura looked over at her friend with a bland look before returning her attention to her advanced chemistry textbook. It was a warm Tuesday afternoon, and on those days both she and the blonde knucklehead would hang out for an hour after class to wait for Sasuke, who was part of the student council, to finish his weekly meeting so they could walk home together.

"I only feel a very minute amount of sympathy for you," she informed him as she highlighted a piece of text. "I would be more empathetic if you actually put in more effort than just showing up to class. Really, it's a problem –"

" – of your own making, yeah, yeah," Naruto finished for her, waving a hand tiredly in dismissal without bothering to lift his head from the cocoon he'd made with his arms to rest his face on. She snorted at him but didn't bother to hit him as she was tempted. He looked miserable enough as it is.

"Ebizu is a tyrant," he whined.

"And you're lazy. Great match, really."

Naruto turned his head towards her and opened one eye, squinting at her from above his arms. "Is that sarcasm?"

"Absolutely."

Rolling his eyes in exasperation, he rolled over to lie on his back. After letting a comfortable silence settle for a few minutes, without looking at her he sighed. "Can you help me?" he asked in a quiet, reed-thin voice.

Sakura lowered her textbook and looked at him contemplatively without saying a word. Her silence was beginning to make him uncomfortable, but he didn't dare meet her eyes. After letting the silence stretch (unnecessarily long, in his opinion), she finally answered him.

"Are you serious?"

Okay, that wasn't the response he was expecting. "Yeah!" he said. "Why wouldn't I be? If I fail this test my life is over!"

"Gee, I don't know, Naruto," she answered him with an annoyed look. Uh-oh. "I mean, you've only asked me for help for practically half of the subjects we've been taught since we met back in the second grade, and while I agree every time, all you do is whine and complain that you don't get it. Which wouldn't be so bad if you hadn't done it before the tutoring session even started every. Single. Time."

Naruto gulped and hurried to sit up, looking beseechingly at her. "I'm sorry, Sakura-chan! You know it's not that I want you to waste your time! You know I really appreciate all your help!"

She huffed at his puppy-eyed look. "You know it's not that I don't want to help you, but you're the one who needs the extra boost, not me. There's only so much I can do for you, so if you're not even going to bother to at least try then neither will I."

Scrambling to his knees, he clasped his hands together in front of his face as he leaned closer to her with the most pleading look he could muster. "Please, Sakura-chan, please please please please pleeeeaaaaassee! I promise I'll try!"

She raised an eyebrow at him. "Just try?"

"I'll work hard, I promise! I won't whine or complain, and I'll pay attention! I promise! I'll be a model student like I've never been before in my life! Just please help me!"

Tapping her lip in contemplation, she thought over how likely his wild promises were (coming form him, they were quite something), and a heartbeat later she relented. "Fine. But I want you to swear that you mean every word that you just said."

Giving her a relieved smile, he beamed at her. "I swear!"

"No whining, no complaining, no interrupting, no inappropriate comments?"

"You won't hear a peep from me."

"No stupid sexual jokes or innuendos?"

"You won't hear them from _this_ mouth," he said, pointing at his smile.

Crossing her arms, she gave him a serious look. "Do you promise to take this seriously and not give up? And only ask relevant questions?"

"I double swear!"

Nodding in satisfaction, knowing that once Naruto gave his word he'd rather die than break his promise, she sat back and started thinking about how they'd go about it.

Sitting back down beside her, happy that she had his back – quite happy, really, as she was widely recognized as the brightest student in their school and was a great teacher despite his less than stellar attitude in the past – he grinned at her. Having Sakura help him was a huge relief. And if he was honest, he'd much rather have her as a teacher, regardless of the subject. He could admit the truth of her earlier words, after all, he really was a bit of a little turd when it came to studying in the past. But despite his reluctance to actually learn, her study sessions still somehow managed to stick and help him pass.

"So," he said jovially. "When do we start?"

Sakura closed her textbook and smiled at him. "Right now."


	2. Chapter 2 - The Basics: DNA

"First off, lets start by pinpointing the gaps in your knowledge."

Naruto laughed nervously as Sakura snatched his bag and opened it, riffling through all his junk to pull out his biology notebook. As she flipped open the front cover, a credit-card sized image of a naked woman, posing provocatively with black censor bars covering her privates, slipped out and fluttered to the ground. Sakura froze when her eyes fell on the image.

A long, tense few seconds passed before she slowly reached for the picture, gave him a deadly glare, and ripped it violently in half before repeating the action several times until the offensive image was no more than confetti. Naruto bit his cheek to keep from protesting. He may not be that smart, but heaven help him if he opened his mouth and try to defend himself to his temperamental female friend.

Giving him one last disgusted look, Sakura glanced down at the first page of his notebook, where she'd personally stapled the course outline to ensure that he wouldn't lose it. After skimming it for a second, taking in the reading schedule and covered topics up to date, she nodded to herself.

"Alright," she said. "Can you tell me the difference between a zygote, an embryo, and a fetus?"

Naruto looked bewildered. "What?"

"I'll take that as a no. Alright, the difference between mitosis and meiosis?"

"Uh…"

"Define 'gestation'?"

"Well, you see…"

Sakura continued to ask him questions, but although she kept her face impassive and practical, Naruto could physically feel her frustration increase with every question.

"Somatic cells, stem cells, germ cells, gametes?"

"Er… kind of?"

"Okay then. What can you tell me about the women's reproductive cycle?"

"It happens every month, causing them to bleed and PMS…?" he trailed off, unsure.

She gave him a deadpan stare. "Do you even know what PMS stands for? And it's a noun, not an adjective."

"Pain in My Side with a side of bitchiness?" he tried to joke. It didn't work; in fact, it earned him a solid smack on the back of the head with his own notebook along with a scathing look and muttered words about being an insensitive ignoramus.

Sighing loudly, Sakura looked to the sky, seemingly asking for patience, before flipping through his notebook. She saw some messily written notes written haphazardly all over the pages, a lot of doodles, and a few conversations between him and Sasuke. Mainly Sasuke telling Naruto to piss off and to pay attention.

Tossing his notebook at him, Sakura rubbed her eyes with the heels of her hands and flopped back onto the grass. Naruto, feeling a little guilty, awkwardly fidgeted with the hem of his shirt as she waited for her verdict. He knew that in some way, he'd failed her. After all, she helped him all these years with his studies but he still couldn't seem to grasp something as basic as taking decent notes. And based on her quizzing, it sure appeared that he might as well not show up in class for all she knew.

He really hated feeling like he'd let her down. Really, _really_ hated it.

After taking a deep breath, Sakura raised her arm and, without looking at him, gestured for him to hand her his notebook. Nervously, he did, and she sat up. She looked at the notebook calmly, lost in thought for a second before she faced him and smacked him upside the head again, this time harder.

"Ow! Sakura-chan, was that really necessary?"

"Yes," she said simply.

"Oh…"

"Why are you hitting the dork and what do I have to do to join in?" asked a new voice with a teasingly smug tone.

The two friends looked up and saw Sasuke walking towards them, his carried bag looking a tad bit heavier than it had at the end of classes earlier. The two of them knew that the extra weight came from the paperwork that he was required to do as part of his club. Sometimes being the vice-president of the student council _sucked_.

"You're out early. You usually aren't done for another half hour or so. And I'm helping Naruto with his biology," Sakura explained.

"It was a short meeting, mainly about getting this annoying paperwork done. No big news there," Sasuke said. "Why the head hits?"

"Because he's an impossible idiot who doesn't even take notes. No wonder he finds it impossible to study if he can't remember what they covered in class in the first place."

"Sakura-chan… that's a bit harsh…" Naruto whined pitifully as he rubbed the spot on the back of his head where she'd hit him.

Sasuke raised an eyebrow at him. "She found the proof of me telling you to pay attention?"

"Shut up."

"I'm not going to coddle you, Naruto," Sakura told him firmly. "Yes, what I said is harsh, and yes, I could have worded it a bit nicer. But it doesn't make it any less true. I've tried wording things nicely in the past, but since that doesn't seem to do the trick, I'll just teach you this time around in the only way I know that will make it stick for sure in your head – painful bluntness."

Naruto hung his head. He really couldn't argue with that logic. His two friends knew as well as him, and better than even his godfather who raised him, that the only way to carve concepts into his mind was to be direct – short and sweet. But with some force behind it. One or two previous teachers tried to teach him in an overly sweet way as if he was a child or mentally retarded, which only made things worse as he felt he was being talked down to more than taught.

Honestly it wasn't such a bad way to learn. It caught his attention and engaged him, weirdly enough. The only downfall in this way of learning was that it was a delicate art. Not many people could teach using that method without sounding patronizing, angry, or as if they were berating him.

Luckily, it seemed Sakura knew how to. Or at least had an inkling.

"So when are you guys going to start studying?" Sasuke asked, giving them a curious look.

"We're starting right now. From scratch," Sakura answered.

"From scratch?" both Sasuke and Naruto asked at the same time, sharing a surprised look between them.

She nodded. "It seems that, presently, Naruto doesn't have gaps in knowledge so much as just a general lack of knowledge. However," she added more gently, seeing Naruto's crestfallen look, "I know that's not quite true. I know for a fact that you have some of this information bouncing about somewhere in your head. We just have to coax it out. So I'm going to start a general review, from scratch, to help get you into the biology mindset gradually; once you get the hang of it we'll go focus on the exam material."

Naruto smiled brightly at her. She may speak so frankly at times that she may sound harsh, but she knew what she was doing. And she believed in him. That meant to him more than anything in the world.

"Alright, give me a pen and take out your textbook," she instructed him.

As he dug around his backpack to do her bidding with renewed vigor, Sasuke turned to her and gave her a bemused look. "You're starting right now?"

Accepting a dark blue pen from the blonde, she flipped open his notebook to a blank page and nodded. "The sooner the better. You can stay and be part of the review session if you like. Or you can go home. We're fine with whatever you decide, but I really want Naruto to do well, and time is of the essence."

Sasuke considered her offer for a minute. On one hand, he was tired, he was doing well in the class, and he just wanted to rest; not to mention he had the annoying club paperwork to do.

On the other hand, he really, really hated paperwork, and it wasn't due for a another few days. Plus his annoying older cousin Shisui was crashing at his place for the time being, and staying with his friends would mean one hour less he'd have to spend with the guy. Not to mention that Sakura's review sessions were damn near legendary. Not that he really needed them, but hey, she was offering and it had been years since she'd done that.

He dropped his bag beside her and sat down before flopping onto his back and rolling to his side, propping up his head with his hand on his cheek.

Getting their answer, Sakura jotted down a few notes before turning to Naruto, who'd slicked back his hair with his hand and put on some old plastic sunglasses with the lenses popped out, sitting straight and looking ready to go.

"…What are you wearing?"

Naruto blinked, taken aback, before laughing bashfully and rubbing the back of his neck self-consciously. "I, erm… haha… I wanted to seem like the role model student I promised I'd be, so… I mean, looking the part is only half the battle, right?"

Chuckling at his silly shenanigans, she removed his sunglasses carefully and set them aside before tussling his hair back into its usual wild style.

"You don't need to look like someone else, you dummy. Just be yourself. And pay attention, that's all."

"Be your attentive self," Sasuke summarized helpfully. "I'm not entirely sure that self is not a myth, but…" he trailed off, rolling quickly to his other side as Sakura aimed to kick him.

"If you stay, then you're going to be helpful and supportive, or so help me, Sasuke, I'll carry you home myself and deliver you to Shisui in a basket with a bow around your neck."

Sasuke shivered involuntarily at her threat, raising his hands in surrender. Naruto snickered at him from Sakura's other side.

"Alright, enough goofing off. Naruto, use the course guideline to find the page where the DNA and Cells chapter starts on your textbook. I know for a fact that you know your nucleic acids – those are the A, G, T, and C parts of DNA – and the basic idea of genes, so I'll skip that."

Naruto obediently did as he was told and passed the book to her. She flipped through the first couple pages before finding what she was looking for. Flipping the book around to make it visible to Naruto, she indicated at a black and white picture with a grey background and black strips of what seemed like short spaghetti bits arranged in pairs. Sasuke repositioned himself, scooting closer to Naruto in front of Sakura's crossed legs to get a better look.

"Do you know what these are?" she asked.

"Uhm… chromosomes, I think."

"You think?" Sasuke scoffed, but shut his mouth at Sakura's glare.

"Yes, they're chromosomes," she smiled at the blonde. "Now here's a question for you that I find a lot of other students don't quite know how to answer: what _are_ chromosomes?"

"Er…" Naruto struggled to answer. He leaned closer to the textbook and looked for the bolded definition on the page, but as soon as he started reading it his mind began to fog up. _Chromosomes are a thread-like structure of nucleic acids and proteins found in…_

"They have something to do with DNA," he answered unsurely but determined to do his share of their deal. He knew his answer was severely lacking, but it was all he could recall.

"Good," Sakura said. She ripped out the page she'd written on, put it beside her and returned his notebook to him along with his pen. "Okay, Naruto, I want you to write either exactly what I say or how you understand what I say in your own words. Okay?" At his nod, she continued. "You're right, chromosomes have to do with DNA. They're a huge component of it, really. What I think a lot of people have a bit of a problem understanding is how the two are connected. So, in the simplest terms I can think of, I'll tell you what that relationship is. Write it down.

"Chromosomes are packages of DNA. Plain and simple. See, DNA, we know, is a long string of genes. If we were able to make it big enough to see with the naked eye, an average strand of DNA would go from one end of a footfall stadium to the other."

"Woah, seriously?" Naruto asked, looking up from his scribbling. Even Sasuke looked intrigued.

"Yep. But the problem lies in the fact that cells are _tiny_. If our DNA is the length of a football stadium, to fit inside a cell we'd have to somehow make it small enough, without altering it, to fit within half an inch or so." She held her fingers slightly apart to drive her point. "How in the hells is a strand of DNA supposed to fit inside a cell, never mind twenty-four _pairs_?"

"Good question…"

To help with her explanation, Sakura reached to her head and plucked out a long strand of her pink hair before digging into her pocket for a penny. Fishing it out, she held it in front of her for the boys to see.

"Say my hair is a DNA strand and the penny the cell it's supposed to go in. My hair doesn't fit inside the coin, does it?" she asked as she placed the single hair on top of the coin. The long ends flopped on either side of it almost comically. "Naruto, how would you think of getting my strand of hair to fit inside the coin?"

Naruto reached for the two things with a concentrated look on his face. He lifted the hair and twirled it in front of his face between his thumb and forefinger, thinking. He absentmindedly noticed that rubbing the pads of his finger together with the hair between them made a sort of knot out of the hair strand. Staring at the rumbled part of the hair, his face lit up as he got an idea.

Rubbing his fingers together in a circular motion, he watched in satisfaction as the ends of the hair strand shrunk as its length was consumed as it formed a larger knot. Grinning, he placed the little knot of hair on top of the penny. It fit perfectly, with space for a couple more on the surface.

"Nicely done," Sakura praised. "Can you tell me in words what you did?"

"I rubbed my fingers together to twist the hair into a little ball." Sasuke rolled his eyes but kept his comments to himself.

"I want you to focus on one word you said – twist. Another way to put it is that you twisted, you coiled the hair strand around itself. And by doing so it became smaller, did it not? That's why it now fits on the penny."

Taking the coin and hair from the blonde, she pocketed the penny and held up the little knot of hair between her nails. "This knot of hair – or, this coil of DNA – is a chromosome. It is DNA wrapped tightly around itself over and over again so that it is small enough to not only fit inside a cell, but also small enough to leave enough space for the other forty-seven chromosomes that must also fit inside it. One thing that you must remember about chromosomes is that it's not just the DNA made into a knot. There's also a few proteins that bind to it so it keeps its shape. Without these proteins, the chromosome will come undone. You follow?"

At the blonde's nod, Sakura smiled in satisfaction and tossed the hair away. "Tell me how you would word what I just said in a single sentence."

Naruto scratched his head with the end of his pen, with a furrow between his brows. "Chromosomes are packages of DNA… they are made of coiled DNA and proteins to help it keep its shape so the package doesn't unwind." The last sentence came out almost as if he was asking.

He looked up cautiously at her, aware that his answer was beyond simple and not all that accurate. However, instead of a scolding as he'd come to expect from most of his teachers, she smiled brightly at him.

"Good! Write that down. We'll worry about the details later; right now I just want you to have a basic understanding. Once you have that the details will be a zilch."

"Cells very small. DNA strand very big. Many DNA must fit inside cell. Answer: coil each until it shrinks enough and stuff it in," Sasuke summarized monotonously.

Sakura rolled her eyes at him but didn't kick him. He smirked at her.

When Naruto was done writing his definition, Sakura continued. "Now, you know chromosomes come in pairs, meaning in sets of two. You should know the answer as to why that is."

Naruto brightened considerably. "I do! Since there are twenty-four types of chromosomes, there are twenty-four pairs, for a total of forty-eight. So if you get half of your DNA from each parent, that means that you got twenty-four chromosomes from your mother and the other twenty-four from your father."

Sasuke looked impressed that Naruto could vocalize that much. His face said as much. Naruto, seeing his look, grinned smugly at the raven-haired boy, who shook his head at him in return.

"Nicely done, Naruto," Sakura said. "I just want to clarify one thing. There aren't different types of chromosomes. They're made of the same stuff and in the same way, the only thing that differences them is how long they are. See?" she pointed back to the image on the book. She indicated to the numbers under each pair. "Chromosome pair 1 is had the longest chromosomes. Pair two is a little smaller. Pair three is smaller than that, and so on and so forth until you get to pair 23, which are the smallest. It also helps chromosomes identify their partners. See how the pairs are identical in size? It would be really weird to have mismatched pairs."

"Question!" Naruto shot his hand straight up over his head. When Sakura gave him the go-ahead, he asked, "Why are only pairs 1 through 23 the same? Why is pair 24 different, and larger than 23?"

Sakura smiled widely, pleased by his question. "Why do you think? What is the single most important pair that we've exclusively studied?"

It took a second for Naruto to file through his memories, but when he found his answer, he snapped his fingers. "The sex pair!"

Sasuke snorted as he tried to choke back his laughter. The other two ignored him.

"Go on."

"Pair 24 has the X and Y chromosomes. They determine the sex of the baby. Two X's make a baby girl, while an X and a Y make a boy."

"Excellent! Is it possible to have two Y chromosomes?"

Naruto shook his head vehemently. "Why not?" Sakura pried.

"Because women only have X chromosomes to pass onto their baby; there is no way for the child to get a second Y chromosome. The father only has one X and one Y chromosome to give, and can only give one of the two – so the baby can only get an X chromosome from the mom, and an X _only_ , and either an X or a Y from its dad."

Sasuke looked impressed that the blonde was able to explain that much on his own, his jaw dropping slightly. Sakura clapped her hands in delight, happily reminding Naruto that she knew that he had it in him while Naruto bashfully scratched his cheek as his face flushed under her praise.

After a shaking himself out of his shock, Sasuke smiled at his friends.

They'd be alright.

* * *

 **I did say I'd focus on the menstrual cycle, but that doesn't mean I won't cover other things. I want to work my way up. Consequently, the three-shot I'd originally envisioned seems to be longer than I thought. Conversations in your own head never seem as long until you start writing them down, haha.**


End file.
